La fork bomb est une forme d’attaque par déni de service contre un système informatique utilisant la fonction fork. Elle est basée sur la supposition que le nombre de programmes et de processus pouvant être exécutés simultanément sur un ordinateur est limité.
Une fork bomb fonctionne en créant un grand nombre de processus très rapidement afin de saturer l’espace disponible dans la liste des processus gardée par le système d’exploitation. Si la table des processus se met à saturer, aucun nouveau programme ne peut démarrer tant qu’aucun autre ne termine. Même si cela arrive, il est peu probable qu’un programme utile démarre étant donné que les instances de la bombe attendent chacune d’occuper cet emplacement libre.
Non seulement les fork bombs utilisent de la place dans la table des processus, mais elles utilisent chacune du temps processeur et de la mémoire. En conséquence, le système et les programmes tournant à ce moment-là ralentissent et deviennent même impossibles à utiliser.
Les fork bomb peuvent être considérées comme un certain type de wabbit (un programme qui s’autoréplique sans utiliser de réseau). (Wikipedia)
Comment se protéger d’une telle attaque sous Linux Unix?
Il faut limiter le nombre de process des utilisateurs. Dans le fichier /etc/security/limits.conf on configure les limitations.
Description du fichier /etc/security/limits.conf
Each line describes a limit for a user in the form:
Where:
an user name
a group name, with @group syntax
the wildcard *, for default entry
the wildcard %, can be also used with %group syntax, for maxlogin limit
« soft » for enforcing the soft limits
« hard » for enforcing hard limits
core – limits the core file size (KB)
core – limits the core file size (KB)
data – max data size (KB)
fsize – maximum filesize (KB)
memlock – max locked-in-memory address space (KB)
nofile – max number of open files
rss – max resident set size (KB)
stack – max stack size (KB)
cpu – max CPU time (MIN)
nproc – max number of processes
as – address space limit
maxlogins – max number of logins for this user
maxsyslogins – max number of logins on the system
priority – the priority to run user process with
locks – max number of file locks the user can hold
sigpending – max number of pending signals
msgqueue – max memory used by POSIX message queues (bytes)
nice – max nice priority allowed to raise to
rtprio – max realtime priority
chroot – change root to directory (Debian-specific)
Se connecter en root et éditer le fichier:
# vi /etc/security/limits.conf
Exemple pour éviter un « fork bomb »:
@student hard nproc 50
@faculty soft nproc 100
@pusers hard nproc 200
Il faut ensuite rebooter le serveur. On peut aussi utiliser ulimit pour la session active.
Cette commande permet de définir différents paramètres (soft : modifiables, et hard : non modifiables) pour la gestion de la mémoire et des processus.
Ainsi, dans notre cas, on utilise ulimit de la façon suivante :
Exemple limiter à 30 process les utilisateurs :
# ulimit -u 30
Pour vérifier et lister les limites prises en compte :
# ulimit -a
…
max user processes (-u) 30
On peut utiliser ulimit pour d’autres paramètres.
Augmenter la stack size. La valeur à indiquer est en Kb.
Si vous avez des programmes qui déclarent de gros tableaux, vous pouvez changer la valeur de la stack size qui est de 8MB par défaut (vous avez besoin d’être super-user) :
# ulimit -s
8192
# ulimit -s 32768
# ulimit -s
32768
Il est aussi possible de fixer les limites soft et hard de façon manuelle. La limite hard n’étant modifiable qu’une seul fois par session, et limitant aussi la limite soft.
Pour modifier la limite soft :
ulimit -S -m 800000
Pour la limite hard :
ulimit -H -m 800000
Sachez toutefois que ces limites ne sont fixés que pour la session courante, il faut donc modifier votre fichier /etc/security/limits.conf
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